I find ai is a terrible TB to list how ai is with each demi: (listed from best to worst in each list)

Good

Erebus

Sedna

UB

Bad

Oak

QoT

Regulus

Torchbearer

Rook

Reg's only good cause of gear which nightmare ai gets a lot of while sedna, UB and Erebus always seem to least be decent with their skill use in addition to good gear. Ai TB over-extends far to often and never knows when to run. Those flaws are only made worse when using rook who's only good if you know when you're over-extending yourself.

Replacemnt midway:in a replacemnt situation a TG AI can end up with skills it does not at all know how to play, all AI are worse then a player when replacing it. the AI of TG gets on my top3 worst. a few days ago i played against a very good rook tower spammer was a close game but enemy team was pulling ahead after 30min i still had 0 kills on the rook which stodd in a circle of 8 towers most of the time then he disconnected (hes team was winning) after that each time i ran into the rook AI i killed him. over and over until the other two players in oponent team left they were losing so bad. the AI rook was built for towerfarming but didnt play like it and died.

tournament:if you play tournament the hard/nitemare then your team mate AI are going to be normal and cannon fodder for the enemy Demigods. tournament is the worst possible place to evaluate AI skills unless on normal, and even then it wont tell you how good the AI demigod will be when replacing a real player.

I had a panth game that start 2v1 all human and the 1 dropped, which was understandable. So it was me as reg and an ally oak vs a loan UB. The ai did alright but at one point it was sitting on full health at an already captured portal flag(zikurat) i ran up to it and started auto attacking and it didn't move or do anything it just let me kill it without taking any action.

Just a funny ai story haven't played with them enough to judge how good they are at each demigod though.

As far as AI's go, I'd say it's definitely most dangerous as Regulus.  As for unclean beast, I think it depends.  If it picks spit, it does great.  If it doesn't, it just plain sucks.  Aside from that, I find it plays Sedna and Oak okay. 

I don't know what you're talking about the Erebus AI playing well.  I find it's definitely the weakest AI by far.  Doesn't know how to use any of its skills effectively and really easy to kill.  I will agree it's terrible as Rook, QoT, and Torch.

The WORST ai is rook. For some reason the AI never runs away as the rook, always studdering a bit, then dieing.

Regulus is the official easy mode demigod so that's not a surprise that the AI can be maybe semi-effective with him. 

I wouldn't agree with TB, it's quite terrible ,it doesn't know when to retreat and often just stands there when there's an Oak or UB pounding on it.

I've actually found AI to be somewhat inconsistent in how terrible it is, especially with Sedna, Oak, and UB. I'm not sure what sort of algorithm they used to determine which ability it picks at what level, I'm guessing it's not always the same. Sometimes I've seen AI Sedna be ok at staying alive, but other times she just dies over and over again. Perhaps there's luck involved, maybe if the AI "decides" to cast the yeti spell often, it won't have mana left over for heal and you can kill it, since yetis are expensive and it's a crap skill.
Also, it might depend on the map, and game mode.

And yes, the AI is horrendous with Rook and Queen, and almost always Erebus.

NO THATS A DUMB KITTY